Digital transmitter

ABSTRACT

An equalizer provided in a digital transmitter compensates for attenuation in a signal channel to a digital receiver. The equalizer generates signal levels as a logical function of bit history to emphasize transition signal levels relative to repeated signal levels. The preferred equalizer includes an FIR transition filter using a look-up table. Parallel circuits including FIR filters and digital-to-analog converters provide a high speed equalizer with lower speed circuitry. The equalizer is particularly suited to in-cabinet and local area network transmissions where feedback circuitry facilitates adaptive training of the equalizer.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/914,350, filed Jun. 10, 2013 which is a continuation of Ser. No.12/942,607, filed Nov. 9, 2010, which is a continuation of U.S.application Ser. No. 12/571,582, filed Oct. 1, 2009, now U.S. Pat. No.8,243,847, which is a continuation of application Ser. No. 11/514,735,filed Aug. 31, 2006, now U.S. Pat. No. 8,254,491, which is acontinuation of application Ser. No. 11/483,971, filed Jul. 10, 2006,now abandoned, which is a continuation of application Ser. No.10/372,630, filed on Feb. 24, 2003, now U.S. Pat. No. 7,099,404, whichis a continuation of application Ser. No. 09/852,481, filed on May 10,2001, now U.S. Pat. No. 6,542,555, which is a continuation of Ser. No.08/882,252, filed on Jun. 25, 1997, now U.S. Pat. No. 6,266,379, whichis a continuation-in-part of Ser. No. 08/880,980, filed on Jun. 23,1997, now abandoned, which claims the benefit of U.S. ProvisionalApplication No. 60/050,098, filed on Jun. 20, 1997. The entire teachingsof the above applications are incorporated herein by reference.

GOVERNMENT SUPPORT

This invention was made with government support, under Contract No.F19628-92-C-0045 awarded by the U.S. Air Force Systems. The Governmenthas certain rights in the invention.

BACKGROUND OF THE INVENTION

The performance of many digital systems is limited by theinterconnection bandwidth between chips, boards, and cabinets. As VLSItechnology continues to scale, system bandwidth will become an even moresignificant bottleneck as the number of I/Os scales more slowly than thebandwidth demands of on-chip logic. Also, off-chip signaling rates havehistorically scaled more slowly than on-chip clock rates. Most digitalsystems today use full-swing unterminated signaling methods that areunsuited for data rates over 100 MHz on one meter wires. Even goodcurrent-mode signaling methods with matched terminations and carefullycontrolled line and connector impedance are limited to about 1 GHz bythe frequency-dependent attenuation of copper lines. Without newapproaches to high-speed signaling, bandwidth will stop scaling withtechnology when we reach these limits.

SUMMARY OF THE INVENTION

Conventional approaches to dealing with frequency dependent attenuationon transmission lines have been based on equalization, either in thetransmitter or the receiver. For example, Tomlinson precoding is used inmodems, and digital equalization in binary communication channels hasbeen suggested in U.S. Pat. No. 4,374,426 to Burlage et al. However,such systems cannot scale to very high data rate binary or multilevelsystems having bandwidths extending from near DC to greater than 100MHz. Above 100 MHz, there is substantial attenuation due to skin effectresistance on conventional transmission lines.

The present invention enables equalizers which can be implemented asdigital filters operating at acceptable clock speeds. For example, athree gigabit per second (Gbps) system can be implemented using 400 Mbpscircuitry. The invention has particular application to nonmodulated,high data rate, binary or multilevel systems as found locally within adata processor cabinet or on a local area network.

In accordance with the present invention, a digital transmittercomprises an equalizer which emphasizes transition signal levelsrelative to repeated signal levels. In particular, a novel equalizergenerates signal levels as a logical function of bit history toemphasize transition signal levels. Preferred implementations define thelogical function of bit history in a look up table.

In preferred embodiments, the equalizer converts an input signal, havingdiscrete signal levels at an input data rate, to an output signal havinga greater number of discrete signal levels at the input data rate. Inparticular, the equalizer generates transmitted signal levels based ontime since last signal transition. A particularly simple implementationis based on whether a current bit is equal to an immediately previousbit.

The clock rates of circuitry can be reduced by multiplexing outputs ofparallel logic circuits operating on different multiple bit inputs togenerate the signal levels. In an adaptive system, the level ofequalization in the transmitter can be modified as a function of signalsdetected at the receiver.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 illustrates a digital communication system embodying in thepresent invention.

FIGS. 2A and 2B illustrate a sample binary pulse train and the resultantfrequency dependent attenuation caused by a transmission line.

FIGS. 3A and 3B illustrate the resistance and attenuation curves for onemeter of 30 AWG, 100 ohm twisted pair transmission line, and FIGS. 3Cand 3D illustrate the resistance and attenuation curves for one meter of5 mil 0.5 oz 50 ohm strip guide.

FIG. 4A illustrates respective plus and minus signals in a differentialsystem and the reduced data eye due to attenuation; FIG. 4B illustratestrailing edge jitter; and FIG. 4C illustrates the data eye withequalization.

FIGS. 5A and 5B illustrate impulse response and frequency response of anequalizing filter embodying the invention, and FIGS. 5C and 5Dillustrate an example input sequence and output sequence from theequalizer.

FIG. 6A illustrates the frequency response of an equalization filterembodying the invention; FIG. 6B illustrates transmission lineattenuation; and FIG. 6C illustrates the combination of equalization andline attenuation.

FIG. 7A illustrates an equalized transmitter signal based on the inputsignal of FIG. 2A, and FIG. 7B illustrates the signal at the receiverresulting from the signal of FIG. 7A to be compared to FIG. 2B withoutequalization.

FIG. 8 illustrates one embodiment of an equalizer of the presentinvention including an FIR filter and digital-to-analog converter.

FIG. 9 illustrates a transition filter for use in a preferred embodimentof the invention.

FIG. 10 illustrates a two tap transition filter embodying the invention.

FIGS. 11A and 11B illustrate a digital to analog converter for use inthe present invention.

FIG. 12 illustrates a preferred multiplexed embodiment of the invention.

FIG. 13 illustrates a transmitter having an encoder, equalizing filter,and driving DAC.

FIG. 14 illustrates a CPU to cache interface embodying the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

A description of preferred embodiments of the invention follows.

The density and speed of modern VLSI technology can be applied toovercome the I/O bottleneck they have created by building sophisticatedI/O circuitry that compensates for the characteristics of the physicalinterconnect and cancels dominant sources of timing and voltage noise.Such optimized I/O circuitry is capable of achieving I/O rates an orderof magnitude higher than those commonly used today while operating atlower power levels.

A system embodying the invention can achieve a four Gbps signaling rateusing 0.5 μm CMOS circuits by controlling and compensating forcharacteristics of the transmission medium, by cancelling timing skew,and through careful management of time and voltage noise.

FIG. 1 shows one channel of high-speed signaling system embodying theinvention. A transmitter module 22 accepts 8-bit parallel data at 400MHz. Each byte is coded into 10 bits (FIG. 13) for band-limiting andforward error correction and transmitted up to 3 m across a singledifferential transmission line. The transmitter pre-emphasizes thesignal to compensate for expected line characteristics. The lossytransmission line as well as package and connector parasitics attenuateand distort the received waveform, and it is further corrupted by noisecoupled from adjacent lines and the power supply. The receiver 24accepts this noisy, distorted signal and its own 400 MHz clock. Thereceiver generates 4 GHz timing signals aligned to the received data,samples the noisy signal, decodes the signal, and produces synchronous8-bit data out.

The availability of 4 Gbps electrical signaling will enable the designof low-cost, high-bandwidth digital systems. The wide, slow buses aroundwhich many contemporary digital systems are organized can be replaced bypoint-to-point networks using a single, or at most a few, high-speedserial channels resulting in significant reduction in chip and modulepinouts and in power dissipation. A network based on 400 MBytes/s serialchannels, for example, has several times the bandwidth of a 133 MBytes/sPCI-bus that requires about 80 lines. Also, depending on its topology,the network permits several simultaneous transfers to take place at fullrate. A group of eight parallel channels would provide sufficient databandwidth (3.2 GBytes/s) for the CPU to memory connection of today'sfastest processors. For modest distances (up to 30 m with 18 AWG wire),high-speed electrical signaling is an attractive alternative to opticalcommunication in terms of cost, power, and board area for peripheralconnection and building-sized local-area networks.

Frequency-Dependent Attenuation Causes Intersymbol Interference

Skin-effect resistance causes the attenuation of a conventionaltransmission line to increase with frequency. With a broadband signal,as typically used in digital systems, the superposition of unattenuatedlow-frequency signal components with attenuated high-frequency signalcomponents causes intersymbol interference that degrades noise marginsand reduces the maximum frequency at which the system can operate.

This effect is most pronounced in the case of a single 1 (0) in a fieldof 0s (1s) as illustrated in FIGS. 2A and B. The figures show a 4 Gb/ssignal (FIG. 2A) and the simulated result of passing this signal across3 m of 24 AWG twisted pair (FIG. 2B). The highest frequency of interest(2 GHz) is attenuated by −7.6dB (42%). The unattenuated low-frequencycomponent of the signal causes the isolated high-frequency pulse tobarely reach the midpoint of the signal swing giving no eye opening in adifferential system and very little probability of correct detection.

The problem here is not the magnitude of the attenuation, but rather theinterference caused by the frequency-dependent nature of theattenuation. The high-frequency pulse has sufficient amplitude at thereceiver for proper detection. It is the offset of the pulse from thereceiver threshold by low-frequency interference that causes theproblem. Later, we will see how using a transmitter equalizer topreemphasize the high-frequency components of the signal eliminates thisproblem. However, first we will characterize the nature of thisattenuation in more detail.

FIGS. 3A-D show the resistance per meter and the attenuation per meteras a function of frequency for a 30 AWG (d=128 mm) twisted pair with adifferential impedance of 100 ohms (FIGS. 3A and 3B) and for a 5 mil(d=125 mm) half-ounce (0.7 mil thick) 50 ohms (FIGS. 3C and 3D)stripguide. For the 30 AWG pair, the skin effect begins increasingresistance at 267 KHz and results in an attenuation to 56% of theoriginal magnitude (−5 dB) per meter of cable at our operating frequencyof 2 GHz corresponding to a bit rate of 4 Gb/s. Skin effect does notbegin to effect the 5 mil PC trace until 43 MHz because of its thinvertical dimension. The high DC resistance (6.8 ohms/m) of this linegives it a DC attenuation of 88% (−1.2 dB). Above 70 MHz the attenuationrolls off rapidly reaching 40% (−8 dB) at 2 GHz. The importantparameter, however, is the difference between the DC and high-frequencyattenuation which is 45% (−6.8 dB).

The effect of frequency dependent attenuation is graphically illustratedin the eye-diagrams of FIG. 4A-C. As shown in the waveform in FIG. 4A,without equalization, a high-frequency attenuation factor of A reducesthe height of the eye opening to 2A-1 with the eye completelydisappearing at A≦0.5. This height is the amount of effective signalswing available to tolerate other noise sources such as receiver offset,receiver sensitivity, crosstalk, reflections of previous bits, andcoupled supply noise. Because the waveforms cross the receiver thresholdoffset from the center of the signal swing, the width of the eye is alsoreduced. As illustrated in FIG. 4B, the leading edge of the attenuatedpulse crosses the threshold at the normal time. The trailing edge,however, is advanced by t_(j). This data-dependent jitter causes greatersensitivity to skew and jitter in the signal or sampling clock and mayintroduce noise into the timing loop.

The waveform of FIG. 4C illustrates the situation when we equalize thesignal by attenuating the DC and low frequency components so allcomponents are attenuated by a factor of A. Here the height of the eyeopening is A, considerably larger than 2A-1, especially for largeattenuations. Also, because the waveforms cross at the midpoint of theirswing, the width of the eye is a full bit-cell giving better toleranceof timing skew and jitter.

Preemphasizing Signal Transitions Equalizes Line Attenuation

Equalization eliminates the problem of frequency-dependent attenuationby filtering the transmitted or received waveform so the concatenationof the equalizing filter and the transmission line gives a flatfrequency response. With equalization, an isolated 1 (0) in a field of0s (1s) crosses the receiver threshold at the midpoint of its swing, asshown in FIG. 4C, rather than being offset by an unattenuated DCcomponent, as shown in FIG. 4A. Narrow-band voice, video, and datamodems have long used equalization to compensate for the linear portionof the line characteristics (Lee, Edward A., and Messerschmitt, DavidG., Digital Communication, Second Edition, Kluwer, 1994). However, ithas not been used to date in broadband signaling with a wide bandwidth(i.e., greater than 100 MHz) over short distances.

We equalize the line using a 4 GHz FIR filter built into thecurrent-mode transmitter. The arrangement is similar to the use ofTomlinson precoding in a narrowband modem (Tomlinson, M., “New AutomaticEqualizer Employing Modulo Arithmetic,” Electronic Letters, March 1971).In a high-speed digital system it is much simpler to equalize at thetransmitter than at the receiver, as is more commonly done incommunication systems. Equalizing at the transmitter allows us to use asimple receiver that just samples a binary value at 4 GHz. Equalizing atthe receiver would require an A/D of at least a few bits resolution or ahigh-speed analog delay line, both difficult circuit design problems. Adiscrete-time FIR equalizer is preferable to a continuous-time passiveor active filter as it is more easily realized in a standard CMOSprocess.

After much experimentation we have selected a five-tap FIR filter thatoperates at the bit rate. The weights are trained to match the filter tothe frequency response of the line as described below. For a 1 m 30 AWGline, the impulse response is shown in FIG. 5A. Each vertical linedelimits a time interval of one bit-cell or 250 ps. The filter has ahigh-pass response as shown in FIG. 5B.

As shown in FIGS. 6A-C, this filter cancels the low-pass attenuation ofthe line giving a fairly flat response over the frequency band ofinterest (the decade from 200 MHz to 2 GHz). We band-limit thetransmitted signal via coding (FIG. 13) to eliminate frequencies below200 MHz. The equalization band is limited by the length of the filter.Adding taps to the filter would widen the band. We have selected fivetaps as a compromise between bandwidth and cost of equalization.

FIG. 6A shows the frequency response of the filter, FIG. 6B shows thefrequency response of the line and FIG. 6C shows the combination (theproduct) for 1 m of 30 AWG cable. The scale on FIG. 6C is compressed toexaggerate the effect. The filter cancels the response of parasitics aswell as the response of the line. The response is flat to within 5%across the band of interest. The filter results in all transitions beingfull-swing, while attenuating repeated bits. FIG. 5D shows the responseof the filter to an example data sequence shown in FIG. 5C(00001000001010111110000). The example shows that each signal transitiongoes full swing with the current stepped down to an attenuated level forrepeated strings of 1s (0s).

FIGS. 7A and B illustrate the application of equalization to the exampleof FIGS. 2A and 2B. FIG. 7A shows the filtered version of the originalsignal and FIG. 7B the received waveform. With equalization the isolatedpulses and high-frequency segments of the signal are centered on thereceiver threshold and have adequate eye openings for detection.

Circuit Implementations

Preferred implementations of the invention include finite input response(FIR) filters, and FIG. 8 illustrates one such implementation. In thiscase, a 5 tap filter has been selected as a balance between higherfractional bandwidth and circuit complexity. With a greater number oftaps, equalization can be obtained at lower frequencies. The presentdesign provides for equalization in a range of 100 MHz to 2 GHz. Byreducing to 2 or 3 taps, the lower end of the range may be no less than500 MHz.

As in a conventional FIR filter, the input D_(i) is delayed insuccessive delay elements 28. However, rather than weighting theindividual delayed signals and summing the weighted signals to obtainthe desired output, the delayed signals are applied to a 5-to-32 decoder32.

One of the 32 output bits from the decoder 32 is high with any inputstate and that high bit addresses a 4 bit word from the 32×4 randomaccess memory 34. The memory 34 is shown to be random access in order toallow for reprogramming of the equalization using a training processbelow. However, the system may be a fixed design which can beimplemented using a read only memory.

The 4 bit output from RAM 34 defines one of the 15 output levelsgenerated by a digital-to-analog converter 36 and applied to thetransmission line 38. Those levels include 0, seven positive levelswhere Dout− is pulled low, and seven negative levels where Dout+ ispulled low. To simplify the implementation, each FIR filter isapproximated by a transition filter implemented with a look-up table asillustrated in FIG. 9. The transition filter compares, in logic elements40, the current data bit D_(i) to each of the last four bits, and uses afind-first-one unit 42 to determine the number of bits since the lastsignal transition. The result is used to look up a 3-bit drive strengthfor the current bit from a 15-bit serially-loaded RAM 44. The drivestrength is multiplied by the current bit with two sets of three NANDgates 46, 48 to generate three-bit high and low drive signals for theDAC.

While the transition filter is a non-linear element, it closelyapproximates the response of an FIR filter for the impulse functionsneeded to equalize typical transmission lines. Making this approximationgreatly reduces the size and delay of the filter as a 96-bit RAM wouldbe required to implement a full 5-tap FIR filter via a lookup table andthe gates 46 and 48.

The transition filter can be simplified even further to the simple logiccircuit of FIG. 10 which operates as a two tap filter. The input signalD_(i) is delayed in a single delay element 50 to produce the signalD_(i−1) The two signals are combined in an exclusive-OR gate 52 todetermine whether the current bit is equal to the immediately previousbit. If so, the lower magnitude output is generated by thedigital-to-analog converter 54. If, on the other hand, there has been atransition since the previous bit, the output is emphasized. Thus, thissimple circuit provides four output levels, two positive and twonegative.

In yet another two-tap embodiment, with a transition, full current driveis used in opposite directions on both sides of the transition. When thesignal value remains unchanged, an attenuated current drive is used.

The circuit design of the DAC used in the FIG. 9 embodiment is shown inFIGS. 11A and B. As shown in FIG. 11A, each DAC module is composed ofthree progressively sized differential pulse generators 56, 58 and 60.Each generator is enabled to produce a current pulse on Dout+ (Dout_) ifthe corresponding H (L) line is low. If neither line is low no pulse isproduced. Depending on the current bit and the three-bit value read fromthe RAM 44 in the filter module, 15 different current values arepossible (nominally from −8.75 mA to +8.75 mA in 1.25 mA steps). Thetiming of the pulse is controlled by a pair of clocks. A low-goingon-clock φ_(i) gates the pulse on its falling edge. The high-true offclock φ_(i+1) gates the pulse off 250 ps later.

Each of the three differential pulse generators is implemented as shownin FIG. 11B. A pre-drive stage 62 inverts the on-clock in inverter 64and qualifies the off-clock with the enable signals in NOR gates 66 and68. A low (true) enable signal, which must be stable while the off-clockis low, turns on one of the two output transistors 70, 72, priming thecircuit for the arrival of the on-clock. When the on-clock falls, thecommon tail transistor 74 is turned on, starting the current pulse. Whenthe off-clock rises, the selected output transistor terminates thecurrent pulse. The delay of the qualifying NOR-gate is carefully matchedagainst that of the on-clock inverter to avoid distorting the pulsewidth.

To enable operation of the equalization circuit at rates in the order ofgigahertz while using circuitry operating only in the order of hundredsof megahertz, the preferred embodiment generates the signal levels bymultiplexing outputs of parallel logic circuits operating on differentmultiple bit inputs.

A block diagram of the multiplexed transmitter is shown in FIG. 12. Thetransmitter accepts 10 bits of data, D₀₋₉, at 400 MHz. A distributionblock 76 delivers 5 bits of data to each of 10 FIR filters 78 offilter/DAC transmitters. The ith filter receives bit D_(i) and the fourprevious bits. For the first four filters this involves delaying bitsfrom the previous clock cycle. The distribution also retimes the filterinputs to the clock domain of the filter. Each filter 78 is a 5-taptransition filter that produces a 4-bit output encoded as 3 bits ofpositive drive and 3 bits of negative drive. These six bits from thefilter directly select which of six pulse generators in the DAC 80connected to that filter are enabled. The enabled pulse generators aresequenced by the 10-phase clock 82, multiplexing their outputs to Out at4 Gbps. The ith pulse generator is gated on by φ_(i) and gated off byφ_(i+1). To meet the timing requirements of the pulse generator, the ithfilter operates off of clock φ_(i+i).

A training sequence may be used to initialize the transmitterpre-emphasis filter at powerup. Training is performed under the controlof a supervisory processor controller 26 that interfaces with thetransmitter on one end of the line and the receiver on the other end viaa low-speed serial scan chain. A preliminary version of a trainingsequence for one channel is as follows:

1. The frequency response of the line is measured. The transmitter iscommanded to turn off precompensation and send an alternating sequenceof 1s and 0s, representing a first bit rate (effective frequency of datatransition). The receiver measures the level of the received signal byusing a feedback transmitter to shift the DC operating point of thesense-amplifiers. The process is repeated at other bit rates(frequencies) to trace out the attenuation curve. For example, bit ratesof R_(max), R_(max)/2, R_(max)/3 . . . may be tested.

2. Based on the attenuation measurements taken in (1), the transmitterequalization is set by programming the FIR filter and/or DAC.

CONCLUSION

Transmitter equalization extends the data rates and distances over whichelectronic digital signaling can be reliably used. Preemphasizing thehigh-frequency components of the signal compensates for the low-passfrequency response of the package and transmission line. This preventsthe unattenuated low-frequency components from interfering withhigh-frequency pulses by causing offsets that prevent detection. Withequalization an isolated pulse at the receiver has the same amplitude asa long string of repeated bits. This gives a clean received signal witha good eye opening in both the time and voltage dimensions.

In one embodiment, we implement equalization for a 4 Gbs signalingsystem by building a 4 GHz, five-tap FIR filter into the transmitter.This filter is simple to implement yet equalizes the frequency responseto within 5% across the band of interest. The filter is realized using0.5 mm CMOS circuitry operating at 400 MHz using a bank of 10 filtersand DACs sequenced by a 10-phase 400 MHz clock. Narrow drive periods arerealized using series gating to combine two clock phases, an on-phaseand off-phase, in each DAC. We have simulated extracted layout of theequalized transmitter driving a load through package parasitics and lmof differential strip guide to demonstrate the feasibility of thisapproach.

The equalizing transmitter described here is one component of a 4 Gbssignaling system we are currently developing for implementation in an0.5 μm CMOS technology. The system also relies on low jitter timingcircuitry, automatic per-line skew compensation, a narrow-aperturereceive amplifier, and careful package design.

The availability of 4 Gbs serial channels in a commodity CMOS technologywill enable a range of system opportunities. The ubiquitous system buscan be replaced by a lower-cost yet higher-speed point-to-point network.A single hub chip with 32 serial ports can directly provide theinterconnection for most systems and can be assembled into moresophisticated networks for larger systems. A single 4 Gbs serial channelprovides adequate data bandwidth for most system components and multiplechannels can be ganged in parallel for higher bandwidths.

A 4 Gbs serial channel can also be used as a replacement technology atboth the component and system level. At the component level, a singleserial channel (two pins) replaces 40 100 MHz pins. A 4 GByte/s CPU toL2 cache interface, for example, (FIG. 14) could be implemented withjust eight serial channels. At the system level, high-speed electricalserial channels are a direct replacement for expensive opticalinterconnect. Using 18 AWG wire, these channels will operate up tolengths of 10 m enabling high-bandwidth, low-cost peripheral connectionsand local-area networks. Inexpensive electrical repeaters can be used tooperate over substantially longer distances.

Even with 4 Gbs channels, system bandwidth remains a major problem forsystem designers. On-chip logic bandwidth (gates×speed) is increasing ata rate of 90% per year (60% gates and 20% speed). The density andbandwidth of system interconnect is increasing at a much slower rate ofabout 20% per year as they are limited by mechanical factors that are ona slower growth curve than that of semiconductor lithography. A majorchallenge for designers is to use scarce system interconnect resourceseffectively, both through the design of sophisticated signaling systemsthat use all available wire bandwidth and through system architecturesthat exploit locality to reduce the demands on this bandwidth.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

1. (canceled)
 2. A computer-readable medium storing a representation ofa circuit comprising: transmitter circuitry to receive input bits and toresponsively generate a serial output signal having a sequence of signallevels; where the transmitter circuitry includes a filter having atleast three taps, each tap driven responsive to respective ones of theinput bits, and the filter is to attenuate signal levels in the serialoutput signal representing no variation between adjacent input bitsrelative to variation between the adjacent input bits, such that signallevels in the serial output signal are emphasized for bits thatrepresent transitions in logic level between adjacent ones of the inputsbits relative to bits that do not represent transitions in logic levelbetween the adjacent ones of the input bits.
 3. The computer-readablemedium of claim 2, where the input bits have a data rate, where thesignal levels have a rate equal to the data rate, and where each inputbit is represented by exactly one signal level in the serial outputsignal.
 4. The computer-readable medium of claim 2, where each signallevel in the serial output signal is to have one of at least fourvalues, including a first value if a first one and a second one of theinput bits each have a first binary state, a second value if the firstand second ones of the input bits each have a second binary state, athird value if the first and second ones of the input bits have thefirst and second binary states, respectively, and a fourth value if thefirst and second ones have the second and first binary states,respectively.
 5. The computer-readable medium of claim 2, where: thetransmitter circuitry is to drive a differential output; the first valueand the second value represent substantially equal drive strength butwith opposite polarity in the differential output; the third value andthe fourth value represent substantially equal drive strength but withopposite polarity in the differential output; the third value representsthe same polarity but greater drive strength in the differential outputthan the second value; and the fourth value represents the same polaritybut greater drive strength in the differential output than the firstvalue.
 6. The computer-readable medium of claim 2, where the transmittercircuitry is to generate each signal level responsive to each tap of theat least three taps.
 7. The computer-readable medium of claim 2, wherefor each one of two possible binary states for each input bit, thetransmitter circuitry generates the serial output signal using one of atleast two possible drive strengths, including a first drive strengthwhen a current bit has a binary state equal to an immediately subsequentbit in the input bits, and a second drive strength when the current bithas a binary state not equal to the immediately subsequent bit in theinput bits.
 8. The computer-readable medium of claim 7, where thetransmitter circuitry is to accept 8 bits of parallel data and is toband limit and encode the parallel data to generate the input bits, toresponsively generate 10 bits of band limited, encoded data as part ofthe input bits.
 9. The computer-readable medium of claim 7, where thesignal levels of the serial output sequence are to vary in frequencyover a range that includes R_(max), R_(max)/2 and R_(max)/ 3, whereR_(max) represents an alternating sequence of ones and zeros in theinput bits.
 10. The computer-readable medium of claim 2, where: thetransmitter circuitry is to generate at least one signal level in theserial output signal for each given input bit; the filter for each giveninput bit is to be driven responsive to three input bits including thegiven input bit and two other input bits; each signal level of the atleast one signal level is to be generated responsive to at least two ofthe three input bits; and the filter is to produce a value of acorresponding signal level in the serial output signal representing novariation between the given input bit and an adjacent input bit which isattenuated relative to a value representing variation between the giveninput bit and the adjacent input bit.
 11. The computer-readable mediumof claim 10, where one of the three bits represents a future bit to betransmitted via the serial output signal, and one of the three bitsrepresents a past bit previously transmitted via the output signal. 12.The computer-readable medium of claim 11, where each of the input bitsis represented by a first signal level and a second signal level in theserial output signal, the first signal level having a first drivestrength if a first bit and a second bit of the inputs bits have acommon binary state, the second signal level also having the first drivestrength if the second bit and a third bit of the input bits have thecommon binary state.
 13. The computer-readable medium of claim 2, whereeach given bit of the input bits is represented by at least one signallevel selected in dependence on whether the given bit represents atransition in logic level relative to an immediately previous input bitand on whether the given bit represents a transition in logic levelrelative to an immediately subsequent input bit.
 14. Thecomputer-readable medium of claim 2, where each given input bit isrepresented by a signal level in the serial output signal thatrepresents emphasis of the given input bit when the given input bitrepresents a transition in binary state relative to a precursor bit. 15.The computer-readable medium semiconductor chip of claim 2, where the atleast three taps include a precursor tap and a post cursor tap.
 16. Thecomputer-readable medium of claim 2, where the transmitter circuitry isprogrammable responsive to measurements taken at a receiver via atraining process.
 17. The computer-readable medium of claim 2, where therepresentation comprises part of a standard-cell library.
 18. Acomputer-readable medium storing a representation of a circuitcomprising: transmitter circuitry to receive input bits and toresponsively generate a serial output signal having a sequence of signallevels; where the transmitter circuitry includes a filter with at leastthree taps, each input bit is represented by no more than two signallevels in the serial output signal, and the transmitter circuitry is touse the filter to emphasize signal levels representing transition inbinary state between adjacent input bits, relative to no transition inbinary state between adjacent input bits.
 19. The computer-readablemedium of claim 18, where the input bits have a data rate, where thesignal levels have a rate equal to the data rate, where each input bitis represented by exactly one signal level in the serial output signal,and where the transmitter circuitry is to generate each signal levelresponsive to each tap of the at least three taps.
 20. Thecomputer-readable medium of claim 18, where for each one of two possiblebinary states for each input bit, the transmitter circuitry generatesthe serial output signal using one of at least two possible drivestrengths, including a first drive strength when a current bit has abinary state equal to an immediately subsequent bit in the input bits,and a second drive strength when the current bit has a binary state notequal to the immediately subsequent bit in the input bits.
 21. Thecomputer-readable medium of claim 20, where each signal level in theserial output signal is to have one of at least four values, including afirst value if a first one and a second one of the input bits each havea first binary state, a second value if the first and second ones of theinput bits each have a second binary state, a third value if the firstand second ones of the input bits have the first and second binarystates, respectively, and a fourth value if the first and second oneshave the second and first binary states, respectively.
 22. Thecomputer-readable medium of claim 21, where: the transmitter circuitryis to drive a differential output; the first value and the second valuerepresent substantially equal drive strength but with opposite polarityin the differential output; the third value and the fourth valuerepresent substantially equal drive strength but with opposite polarityin the differential output; the third value represents the same polaritybut greater drive strength in the differential output than the secondvalue; and the fourth value represents the same polarity but greaterdrive strength in the differential output than the first value.
 23. Thecomputer-readable medium of claim 20, where the signal levels of theserial output sequence are to vary in frequency over a range thatincludes R_(max), R_(max)/2 and R_(max)/3, where R_(max) represents analternating sequence of ones and zeros in the input bits.
 24. Thecomputer-readable medium of claim 18, where: the filter for each inputbit is to be driven responsive to three input bits including the giveninput bit and two other input bits; each signal level of the at leastone signal level is to be generated responsive to at least two of thethree input bits; and one of the three bits represents a future bit tobe transmitted via the serial output signal, and one of the three bitsrepresents a past bit previously transmitted via the output signal. 25.The computer-readable medium of claim 18, where each given input bit isrepresented by a signal level in the serial output signal thatrepresents emphasis of the given input bit when the given input bitrepresents a transition in binary state relative to a precursor bit andby a signal level in the serial output signal that represents emphasisof the given input bit when the given input bit represents a transitionin binary state relative to a postcursor bit.
 26. The computer-readablemedium of claim 18, where the transmitter circuitry is programmableresponsive to measurements taken at a receiver via a training process.27. The computer-readable medium of claim 18, where the representationcomprises part of a standard-cell library.
 28. A computer-readablemedium storing a representation of a circuit comprising: transmittercircuitry to receive input bits and to responsively generate adifferential, serial output signal having a sequence of signal levels;where the transmitter circuitry includes a filter with at least threetaps, each input bit is represented by no more than two signal levels inthe differential, serial output signal, the transmitter circuitry is touse the filter to emphasize signal levels representing transition inbinary state between adjacent input bits, relative to no transition inbinary state between adjacent input bits, and the signal levels of thedifferential, serial output sequence are to vary in frequency over arange that includes R_(max), R_(max)/2 and R_(max)/ 3, where R_(max)represents an alternating sequence of ones and zeros in the input bits.29. The computer-readable medium semiconductor chip of claim 28, wherethe at least three taps include a precursor tap and a post cursor tap.30. The semiconductor device of claim 28, where the transmittercircuitry generates each signal level responsive to each tap of the atleast three taps.
 31. The computer-readable medium of claim 28, wherethe representation comprises part of a standard-cell library.